The present invention relates to photochromic lenses and more particularly to gradient photochromic lenses. The present invention also relates to a method of preparing gradient photochromic lenses or inhibiting photochromic activity in a lens having a photochromic dye therein.
Synthetic plastic photochromic lenses are well known in the art. Such lenses include a plastic host and a photochromic dye either disposed in the host material during the shaping or by imbibing from a solution subsequent to lens shaping. Glass lenses having gradient photochromic properties are also well known in the art. Such lenses are generally prepared by mixing a latent photochromic material with the glass composition during the lens manufacturing process and then selectively activating portions of the lens to provide gradient photochromic activity. Although the lens contains a uniform concentration of latent material, the concentration of material converted from the latent condition to a state capable of acting in response to radiation, such as ultraviolet light, varies across the lens surface.
Dyeing, or tinting as it is frequently referred to, of plastic lenses for aesthetic and sunscreening purposes is well known to those skilled in the art. Such dying processes include those using other photochromic or non-photochromic dyes. Conventionally, the dyes are either uniformly dispersed throughout the lens, when a uniform lens character is desired, or dispersed in the lens providing portions having different concentrations in order to provide special effects such as gradient lenses. It is also known to apply one dye uniformly to a lens and a second dye in a non-uniform manner in order to vary the color characteristics of the lens in different portions thereof. Photochromic dyes in combination with non photochromic are also known.
The propensity of photochromic materials to fatigue, i.e. cease to shift between the inactive and active states, after prolonged exposure to activating radiation is well known. It is also known that lenses fatigue in two different conditions. The first condition is where fatigue produces a permanently activated material. This condition occurs in glass compositions containing silver halides. The second condition is where the fatigue produces a permanently inactive material. The latter condition is known to occur with certain organic dyes. Photochromic lenses having the property of fatiguing in the second condition are preferred. Since potentially active photochromic particles previously screened from activating radiation by other photochromic particles converted to the active state may begin to function photochromically, the life of the article is extended beyond that which it would normally have if fatigue occurred in the first condition.
U.S. Pat. No. 3,666,352 issued May 30, 1972 and discloses that the photochromic activity of mercury thiocarbazones may be prolonged (fatigue resistance) by using a uniform U.V. absorber without affecting the photochromic activity.
U.S. Pat. No. 3,212,898 issued Oct. 19, 1965 and discloses U.V. absorbers to prevent fatigue of photochromic articles by shielding the photochromic material from an extraneous amount of U.V. light .